Neither Russia nor Kazakhstan has acknowledged any accident but state weather service Roshydromet said “extremely high pollution” of ruthenium 106 had been found in samples from two meteorological stations in the southern Ural mountains region in late September and early October.

Agrayash weather station, located around 20 miles from a huge nuclear reprocessing plant known as Mayak, recorded pollution levels 986 times those of the previous month.

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Shocking levels of radioactive pollution have been recorded in western Russia

At the nearby Novogorny station they were 440 times higher.

The Mayak plant accounts for half of Russian exports of radioactive isotopes although it has denied being the source of increased level of ruthenium 106.

The Mayak site was the location for the third-most serious nuclear accident ever recorded, known as the Kyshtym disaster, which prompted the evacuations of around 10,000 people in 1957.

France’s IRSN, a nuclear safety institute, ruled out the possibility of an accident in a nuclear reactor and believes the material it detected is more likely to have been released from a nuclear fuel treatment site or centre for radioactive medicine.

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The Mayak nuclear plant is located close to Russia's southern border with Kazakstan

The institute added that the pollution, even at one thousand times higher than normal levels, would not be sufficient to cause an impact on human health or the environment in Europe.

Ruthenium-106 is a product of splitting atoms in a reactor, and is also used in certain medical treatments. It does not occur naturally.

Jean-Christophe Gariel, the director for health at the IRSN, said the responsibility for identifying the source of the nuclear cloud was with the Russians.

He added: “The matter is closed as far as France is concerned. It’s not a problem for France.